For both consumer and commercial trailer use a safety check is required to ensure that the trailer systems, lights, brakes, etc., are working properly before the trailer is pulled onto a road and into traffic. Trailer and truck plug wiring is often exposed to harsh elements, road dirt and debris that can cause damage to the wires and failure of the electrical circuit needed to operate a particular trailer electrical system. The testing of the trailer system typically requires two people—one to manually engage the circuits from within the towing vehicle such as turning on the lights, turn signals or pressing the brake pedal, and one person to stand behind the trailer and observe whether the circuits are visibly working such as brake lights illuminating. Often when a circuit is found to be not working it takes considerable time, effort and expertise to determine if the problem is with the trailer system or the truck plug. There currently are products in the marketplace that can be attached to an idle trailer's plug and using an external battery source (not the towing vehicle) test the circuits of the trailer to determine any faults within the system. However, these test units are large, heavy and require a 12 volt battery or connection to a 110-120 volt electric power receptacle to provide the power needed to test the trailer system's circuits. Many of these systems also do not provide any diagnostic tools within the system nor are they capable of testing the towing vehicle's plug. These type of testing units are predominately used by truck/trailer rental yards. There are also products in the marketplace that can be installed onto the trailer body itself and then are wired to the trailer wire harness that is connected to the trailer plug. These type of units use the towing vehicle's 12 volt power supply to test the trailer systems. These systems can be wired or wireless such as U.S. Pat. No. 6,525,654 and they require that a separate display device be installed within the cab of the towing vehicle such as a truck/tractor so that the results of the testing and monitoring system can be viewed by the driver. These systems require installation and expertise by the user to correctly and properly wire the transmitter unit to the trailer wiring harness and install the receiver display unit inside the towing vehicle. This level of installation expertise is far beyond the knowledge and skills of the average towing vehicle user or owner. These type of systems are not portable or mobile to be easily and quickly installed and used with multiple towing vehicles and trailers. They also are not capable of testing the towing vehicle's trailer towing plug. It would be highly desirable to have a device that can test a system circuit on both the trailer and struck side of a plug. It would also be highly desirable to be able to monitor those systems in real time while the trailer is being towed so that any fault of any of the electrical circuits within the trailer system could be immediately known and safety precautions taken to ensure the safety of the passengers in the tow vehicle as well as those on the road in the vicinity of the trailer. There are no industry standards for this type of trailer system monitoring from within the towing vehicle nor are there systems provided by truck/tractor manufacturers as standard equipment to provide this type of trailer safety monitoring feature and functionality. There is a need for a simple, universal, inexpensive and highly portable and mobile testing and monitoring system that can be quickly plugged into industry standard plug types without any expertise and easily monitored with a highly available off-the-shelf hand held display unit that can easily fit in your pocket or be displayed in the cab of a towing vehicle. A highly desirable unit would be quickly and easily plugged and unplugged from one towing vehicle or trailer to another and automatically test the trailer systems and vehicle towing plug before using the trailer, diagnose any broken circuits or faults and then monitor the trailer systems in real time while the trailer is in use. An ideal ‘plug and play’ device would be compatible with industry standard 7-pin, 7-blade, 6-way round, 4-way round, 5-way flat and 4-way flat wiring adapter plugs without any customization of any kind making the product highly useful for both the consumer and commercial user. A highly desirable version of a device would also be compatible with the NATO military 12-pin plug.